Powder coating compositions are solid compositions that generally comprise a solid film-forming resin, usually with one or more pigments and, optionally, one or more performance additives such as plasticizers, stabilizers, flow aids and extenders. The resins are usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding crosslinking agent (which may itself be another film-forming polymer). Generally, the resins have a Tg, softening point or melting point above 30° C.
Conventionally, the manufacture of a powder coating comprises melt-mixing the components of the composition. Melt-mixing involves the high speed, high intensity mixing of dry ingredients and then the heating of the mixture to an elevated temperature—above the softening temperature of the resin but below the curing temperature—in a continuous compounder to form a molten mixture. The compounder preferably comprises a single or twin screw extruder as these serve to improve the dispersion of the other ingredients in the resin as the resin melts. The molten mixture is extruded, typically rolled in the form of a sheet, cooled to solidify the mixture and subsequently crushed (pulverized).
Such processing is then generally followed by a sequence of particle sizing and separation operations—such as grinding, classifying, sifting, screening, cyclone separation, sieving and filtering—that precede the application of the powder to a substrate and the heating of that powder to melt and fuse the particles and to cure the coating. The main methods by which powder coatings are applied include fluidized-bed and electrostatic fluidized—bed processes and electrostatic spray processes in which the powder coating particles are electrostatically charged by a spray gun and directed onto an earthed substrate.
Each of these application methods requires the powder to have a specific particle size distribution. For example, most commercial electrostatic spray devices can only apply particles having mean particle sizes up to 120 μm; more optimal particle size ranges for such apparatus are in the range from 15 to 75 μm, and ideally preferably from 20 to 45 μm. In separating out the undesirable large and small particles from the powder particles having the desired size distribution, the various particle separation techniques obviously generate waste powder which must either be downgraded or disposed of.
Studies of the breakage of materials into particles such as KONA Vol. 21, (2003), pp. 88 have demonstrated that, whilst material properties related to comminution—such as modulus, hardness and strength—are generally measured and defined on the macroscopic scale, the microscopic structure of the materials is determinant. The initiation of fractures in a material is believed to occur through the presence of microscopic weaknesses; those on the perimeter of the impact contact area suffer the greatest strain during impact, leading to fracture of the material.
That function which characterizes the size distribution of particles from a comminution process is generally considered to be log-normal; it derives from the statistical probability of the breaking of the weakest link in a chain. Such a function was described inter alia by Weibull in A statistical distribution function of wide applicability Journal of Applied Mechanics, 9, 1951, 293-297:
      P    B    =      1    -          exp      ⁢              {                  -                                    z              ⁡                              (                                  σ                                      σ                    s                                                  )                                      m                          }            wherein:
PB is the probability of breaking a chain which consists of z links of strength σs;
σ is the load applied; and
m is a parameter of the probability distribution, unrelated to any physical property of the material.
According to this function, an increase in the number (z) of weaknesses within a material and decrease in strength (σs) of those weaknesses will result in a higher breakage probability. Therefore, the deliberate formation of weaknesses within the material will act to increase the probability of breakage.
EP-A-0887389 and EP-A-0887390 (Morton International, Inc.) describe methods for producing a powder coating in which direct particle production is enabled by the introduction of gas cells within the powder coating precursor or the extrudate. In these methods a stream of a powder coating precursor including at least one resin and at least one additional ingredient is contacted with a process fluid effective to reduce the viscosity of the powder coating precursor during its extrusion. In a first embodiment, the extrusion of the precursor results in the evaporation (expansion) of the process fluid and thereby results in the direct atomisation of the extrudate into particulate form. In a second embodiment, that evaporation yields a foamed or friable mass of extrudate which must then be subjected to grinding to form particles.
In EP-A-0887389 the process fluid is characterized as being in the form of a supercritical fluid or a liquefied gas whereas in EP-A-0887390 the process fluid is in the form of a gas; these harsh fluid conditions are required to plasticize the resin to thereby reduce the viscosity of the powder coating precursor. Therefore, although these two citations purport to allow processing of the powder coating precursor at lower temperatures, this effect is compensated by the energetic costs required to first elevate the temperature of the process fluids such that they can achieve vapour or supercritical conditions within the extruder. Furthermore, any process fluids that are to be recycled after they exit the extruder must be subjected to pressure boosting at a further energetic cost. As the process fluids are added in relative amounts of up to 90% by weight of the resin, these energetic costs may be significant.